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AGNT40 KWNM 172013
MIMATN

Marine Weather Discussion for N Atlantic Ocean
NWS Ocean Prediction Center Washington DC
413 PM EDT Thu Aug 17 2017

.FORECAST DISCUSSION...Major features/winds/seas/significant
.weather for the North Atlantic Ocean W of 50W from 30N to 50N.

Overnight, as an upper-level low over far southeast Canada moves
east, a ridge axis aloft becomes present over the waters from
north to south. High pressure will be located over much of the
offshore waters Friday, save for weak low pressure just east of
the offshore waters along a dying frontal boundary off of the
Virginia and Carolina coasts. As a cold front approaches the
northern waters from the west and northwest late Friday night
and Saturday, the low-level gradient will increase particularly
over the waters off of New England.

GFS and UKMET guidance both suggest low-level winds increasing
to around 25kt in the tightening gradient overnight Friday night
in the offshore waters off of the New England coast, and this is
despite some stability given the cooler waters there. In fact,
GFS model soundings have an inversion around 950mb at which from
ANZ815 northeast through ANZ805 and ANZ800 winds at the top of
the inversion are forecast as high as 40kt. The temperature
profile appears to be much too stable for those types of winds
to mix down to the water level, but 25kt appears reasonable, and
even the ECMWF, while a little lighter in ANZ815, does provide
for a solid chance of 25kt winds farther northeast. As such,
will lean toward the model consensus and forecast winds
increasing to around 25kt in a good part of the NT1 waters
overnight Friday into Saturday.

As the surface low associated with the front moves north into
Canada, the surface gradient slackens Saturday night into
Sunday, and the front weakens and eventually stalls as it moves
south. Wind speeds will be well below hazards late in the
weekend and early next week as a result. Surface low pressure
that strengthens as it moves northeast away from the Great Lakes
early next week will eventually tighten the surface gradient
once again, and particularly from the mid-Atlantic north, by
Wednesday. While this is well at the end of the forecast period
or just beyond, interestingly, the sounding profiles and
instability parameters off of the New England coast seem quite
similar then to what should transpire overnight Friday into
Saturday, around 35kt at the top of the inversion likely
translating to around 25kt at water level especially over a good
portion of the NT1 waters. The UKMET is a little stronger,
possibly suggestive of near 30kt then offshore, but that is even
stronger than what the 30-meter GFS shows at that point so will
tend to cap winds around 25kt then for now. If the latest ECMWF
is correct, a 984mb low just northwest of Maine would have the
potential to provide for winds around gale force in and near
ANZ800 on Wednesday. It is much too early for this based on one
model run. At this point, chances of any gales next Wednesday
north are non-zero, but the digital output at the far end of the
forecast will reflect conditions then near the model consensus,
to around 25kt.

.SEAS...Wavewatch guidance was generally used throughout the
this forecast period, and with the GFS having slightly higher
winds overnight Friday into Saturday compared to the ECMWF, likes
the slightly higher Wavewatch waves. Well out in time, toward
Wednesday, used the Wavewatch but adjusted in much of the NT1
waters in particular around a foot. At that time, with the
stronger low, the ECMWF waves suggest values around ten feet.
Capped the waves, for now, around 8 feet as a compromise between
the Wavewatch and the ECMWF.

.EXTRATROPICAL STORM SURGE GUIDANCE...N/A.

.WARNINGS...Preliminary.

.NT1 New England Waters...
     None.

.NT2 Mid-Atlantic Waters...
     None.

$$

.Forecaster Figurskey. Ocean Prediction Center.



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